For dual encoder loops with Compax3-EPL with ACR-EPL controller, the setup is slightly different from the standard C3 EPL setup instructions.
For linear encoder systems, you cannot use increments for the drive configuration. Use mm or inches for the drive configuration for the units. For applications with two rotary encoders, increments, mm or in is ok, though the math is easier in terms of mm or in. What is passed across the EPL is the secondary encoder's counts so the controller doesn't know the difference between the drive being in mm, in or increments.
Users are provided a numerator and denominator for both the Drive Configuration's units and also the Load Mounted encoder's units. As an example, if you had a 5mm lead ballscrew and 7:1 gearhead and a 5um linear encoder, set the drive's units to mm, numerator to 5, denominator to 7. For signal source, linear quadrature type encoder; numerator & denominator to 1 and the linear resolution to 20um (pre quad). The numerator and denominator for the signal source is in case the load mounted encoder is connected through belts & pulleys to the machine; obviously for a linear encoder this is directly on the load.
Then for the signal source setup, set the encoder resolution to the pre-quad resolution (linear or rotary). Note for linear encoders, most are specified in post-quadrature resolution.
There is also a field for Maximum Position difference to motor feedback for backlash/hysteresis to prevent dither and lag.
Helpful Objects in the Optimization screen in C3 ServoManager
Actual Position is 680.5 in user units
Load mounted encoder input is under Inputs in the Status Values (not position) and is object 680.10. Even if you Invert Direction Polarity under SignalSource in the configuration, it won't show in the optimization screen when looking at the direction of 680.5 vs 680.10. Look at 680.20:
Position Difference Load Control (unfiltered) is 680.20 and is under Position in the Status Values
Set the maximum speed to a low speed value until drive configuration is verified by commanding short low-speed moves.
For final setup, set maximum speed to the LESSER of the ballscrew critical speed (function of length & diameter of the ballscrew) OR the maximum speed of the linear encoder. Example, Renishaw's 0.1um linear encoder is rated at .4m/s; this is a function of how fast the optoelectronics within the readhead can read the linear encoder scale clearly before edge separation degrades.
Before enabling the load mounted encoder, confirm in the Optimization screen that Position Difference Load Control (unfiltered) [680.20] isn't increasing when jogging the motor. If it's divergent, enabling load mounted encoder will cause a runaway condition. However you do not need to re-wire the linear encoder; in the Load Mounted encoder setup (under Signal Source), change Invert Direction Polarity to ON:
Enable load control and jog in the optimization screen for the CompaxEPL in Setup. When you stop jogging, if the motor doesn't stop, then the scaling isn't correct; the rotary motor will be commanded to turn until the linear encoder position is reached.
Note: After enabling the Compax3-EPL using the Optimization screen's Setup mode, disable this mode and cycle power or perform a DRIVE RES to start control from the EPL controller.
Note: Object 680.10 reports the linear encoder position in millimeters, even though in the C3 optimization screen the units says revs.
The ACR-EPL controller parameter P38144 EPLD position reports the position change of the load mounted encoder. In ACR-View, set the feedback to the pre-quad resolution. Here an E83 encoder (1000 line pre-quadrature) is being used: